Electrostatic support systems or electrostatic chucks have been used to support a substrate, such as a semiconductor wafer, during chemical vapor deposition, sputtering, etching, and other processes. Electrostatically clamping the substrate to the support offers several advantages. The substrate is held against the support by applying a voltage to the support and inducing an image charge on the wafer, with the different potentials attracting the wafer to the support surface. Unlike systems with perimeter clamping rings, electrostatically clamping the substrate to the support leaves the entire upper surface of the substrate exposed for processing. The risk of surface contamination due to the mechanical clamp is also eliminated. In addition, the entire substrate is tightly clamped to the surface of the support by the electrostatic attraction, improving the heat transfer between the substrate and the support. Retaining the substrate or wafer at a substantially constant temperature during processing is important for controlling the chemical process, obtaining process uniformity, and preventing damage to the integrated circuitry already formed on the substrate.
Electrostatic support systems typically include electrodes mounted to a cooled support body. An inert gas, such as helium, is introduced into the space between the substrate and the support surface to facilitate cooling of the substrate. A dielectric coating typically covers the electrodes so that clamping forces exceeding the backing pressure of the heat transfer gas may be obtained. During processing, the substrate is positioned on the dielectric coated electrodes such that the electrodes are isolated from the plasma and process gases. A separate dielectric ion focus ring typically encircles the electrodes to insulate the sides of the electrostatic support from plasma and ensures the electrostatic potential contours remain constant across the entire surface of the wafer so that the center of the substrate and the regions near the edge of the substrate may be exposed to a substantially uniform distribution of the process gases. If the electrostatic support does not include an ion focus ring, electrostatic potential contour lines and ion flux will drop off at the edge of the wafer such that the edge of the wafer is not exposed to a uniform distribution of the process gases. A dielectric ion focus ring ensures the electrostatic potential contours are held constant at the edge of the wafer and reduces the focusing of the ions onto the wafer edge. The upper surface of the focus ring is typically parallel with or below the upper surface of the wafer.
While this ion focus ring provides some protection for the electrodes and underside of the substrate, the separate dielectric ring is thermally uncontrolled because of poor thermal coupling between the different materials of the ceramic ring and the metallic electrode base. Thus, the separate ion focus ring often has an adverse effect on the uniform heat transfer profile between the substrate and the electrostatic support. The use of the ion focus ring also limits the total area of the electrodes for an electrostatic chuck of a given size. If the tolerances of the ion focus ring and electrode base and the assembly of the components are not strictly monitored, the ion focus ring may interfere with the proper placement of the substrate on the electrode base.
An electrostatic support assembly which maximizes the uniform heat transfer between the substrate or wafer and the support assembly is desirable. Increasing the area available for the electrode such that the electrode may extend to the outer edge of the substrate is also desirable. Providing a support assembly which facilitates the accurate placement of the substrate on the support is also desirable. Minimizing the number of components of the assembly also offers the advantages of improved quality and reduced cost.